Intracellular signaling via 3′,5′-cyclic guanosine monophosphate (cGMP) plays an important role in many fundamental physiological processes including regulation of vascular tone, renal and cardiac function, immune responsiveness, thrombocyte activation, retinal phototransduction and bone growth. Because the level of endogenous cGMP modulates the aforementioned biological processes, molecules that regulate cGMP synthesis serve as natural targets for new drug discovery and therapeutic development. Guanylyl cyclases are such molecules.
More specifically, cGMP is produced by the catalysis of guanosine triphosphate (GTP) by one of two families of guanylyl cyclase enzymes: the particulate guanylyl cyclases (pGCs) and the soluble guanylate cyclases (sGCs). The pGCs generally exist as homodimers comprising an extended intracellular domain which includes the catalytic domain, membrane-spanning regions, and extracellular ligand-binding domain. In contrast, the sGCs are intracellular heterodimeric molecules containing a prosthetic heme group which can be activated by nitric oxide. Upon formation of the NO-heme complex, sGCs undergo conformational changes resulting in large increases in catalytic activity. The cGMP thus produced can regulate a number of effector molecules including protein kinases, phosphodiesterases and ion channels.
Of particular relevance to a host of therapeutic indications is the family of particulate guanylyl cyclases activated by natriuretic peptides. Natriuretic peptides are cyclic peptide hormones 28-32 amino acids long which are synthesized as longer preproproteins and processed to yield the mature peptides. Examples of natriuretic peptides include: A-type or atrial natriuretic peptide (ANP), which is released from the heart, urodilatin, the differentially processed form of ANP produced by the kidney, B-type natriuretic peptide (BNP), which is synthesized in the ventricular myocardium and C-type natriuretic peptide (CNP), which is produced by a number of cell types including endothelial cells and chondrocytes (Potter, et al Endocrine Reviews, 27:47, 2006).
One membrane-bound GC bound by ANP, urodilatin and BNP is the natriuretic peptide receptor A (also known as NPRA). Activation of NPRA by these hormones leads to a variety of physiological responses including vasorelaxation, natriuresis, diuresis, lipolysis, inhibition of cardiac hypertrophy and ventricular fibrosis, inhibition of the renin-angiotensin aldosterone system, and inhibition of sympathetic nerve activity. CNP, on the other hand, serves as a potent agonist for natriuretic peptide receptor B (NPRB). CNP-dependent activation of NPRB can also lead to vasodilation as well as stimulation of long bone growth. All of the natriuretic peptides have very short plasma half lives ranging from approximately 2-20 minutes. One reason for their rapid turnover is that they are degraded by proteases such as neutral endopeptidase (NEP), meprin A and dipeptidyl peptidase IV. Moreover, ANP, urodilatin, BNP and CNP also bind to the non-guanytyl cyclase clearance receptor, natriuretic peptide receptor C(NPRC). Binding to NPRC results in internalization of the peptides subsequent lysosomal degradation. See, e.g., Cohen et al., J. Biol. Chem., 271:9863 1996.
NPRA has been shown to play an important role in the regulation of cardiorenal function. Activation of this receptor by ANP and BNP leads to a reduction in cardiac filling pressures, decrease in afterload, diuresis and natiuresis and inhibition of sympathetic and neurohormonal systems such as the renin-angiotensin-aldosterone pathway. Extended NPRA activation has cardiac antihypertropic and anti-fibrotic effects. ANP and BNP are produced by the heart in response to stress and stretch and are elevated in patients with heart failure. NPRA contains an intracellular GC domain and exerts its effects through the production of cGMP. However, as with many single transmembrane hormone receptors identification of small molecule agonists using conventional approaches have not been successful.
Recombinant forms of NPRA ligands have been approved for treatment of acute decompensated heart failure. However, these recombinant ligands have very short half-lives (typically 20 minutes or less) and thus must be administered by extended IV infusion. The recombinant form of BNP (Nesiritide) was approved in the U.S. in 2001 for the treatment of acute decompensated heart failure. Recombinant human ANP (Carperitide) was approved in Japan in 1995 for the same indication and recombinant urodilatin (Utaritide, renal form of ANP) is currently in clinical trials. Review of Nesiritide data submitted to the FDA during the approval process has further revealed significant safety concerns more particularly, increased mortality and reduced renal function, with the administration of recombinant BNP. While many of these safety concerns have subsequently been thought to be speculative, there remains a question as to the potential of the recombinant natiuretic peptides to induce adverse events. As such, the use of the currently approved compositions has been limited to an acute indication and there remains a need for additional therapies targeting the NPRA pathway for managing heart failure in a more chronic setting. A therapeutic intervention leading to the activation of the NPRA pathway that would allow for the treatment of both acute decompensated heart failure with a single administration and chronic heart failure with weekly or monthly injections would greatly benefit patients suffering from these serious conditions. To date such a therapy remains to be found.
Although efforts to modify natriuretic peptides and/or antagonize GC receptors have yielded some therapeutic compositions for use in acute decompensated heart failure, these efforts have failed to produce a robust therapeutic application for chronic heart failure. Thus, there is a current and continuing need to develop molecules that regulate pGCs, which are safe, have superior stability in vivo and effectively modulate the activation of the natriuretic peptide system.